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Primes p such that 2*p-5 is composite.
1

%I #15 Sep 08 2022 08:45:48

%S 7,13,19,31,37,41,43,61,67,73,79,83,97,103,107,109,113,127,139,151,

%T 157,163,167,173,181,191,193,199,211,223,229,239,241,251,269,271,277,

%U 283,293,307,313,317,331,337,347,349,359,367,373,379,397,409,419,421

%N Primes p such that 2*p-5 is composite.

%H Vincenzo Librandi, <a href="/A167462/b167462.txt">Table of n, a(n) for n = 1..1000</a>

%t Select[Range[4, 3000], PrimeQ[#]&& ! PrimeQ[2 # - 5] &](* _Vincenzo Librandi_, Sep 15 2013 *)

%t Select[Prime[Range[100]],CompositeQ[2#-5]&] (* _Harvey P. Dale_, May 16 2016 *)

%o (Magma) [p: p in PrimesInInterval(4,600)| not IsPrime(2*p-5)]; // _Vincenzo Librandi_, Sep 15 2013

%Y Cf. A063909.

%K nonn,easy

%O 1,1

%A _Vincenzo Librandi_, Nov 04 2009

%E Corrected (229 inserted) by _R. J. Mathar_, Nov 30 2009

%E Corrected (3 deleted, as 2*3-5 = 1 and 1 is not composite) by _Harvey P. Dale_, May 16 2016